Active site, concept

Lahaye J., Dentzer J., Soulard P., Ehrburger P. Carbon gasification The active site concept. In Fundamental Issues of Control of Carbon Gasification Reactivity, Lahaye J, Ehrburger P. Ed., Academic Publishers, London, 1991, p. 143-158... 

The initial collective electronic theory of the fifties, in its simplest form, implied that the electrons and holes-controlled by the bulk structure—of the catalytic solid are available for reactants anywhere on the surface. It largely ignored the geometric factor inherent in Taylor s active site concept or in Balandin s Multiplet Theory. 

The selection on an empirical basis of collective-electron factors or active-site concepts or some combination thereof in order to account for the activity of surfaces in catalysing various processes has obvious disadvantages. Possibilities for a more systematic approach to the integration of collective-electron and localised-state aspects of surface structure have developed from theoretical treatments of intrinsic and extrinsic surface states, respectively. Models based on such developments, by reason of their relative novelty, have not yet been as widely applied as collective electron or active-site models to interpret catalytic activity of various surfaces and still less to considerations of sensitivity to irradiation. However, an abbreviated consideration of such surface state models is deemed essential here both as a basis for assessing their possible relevance in the explanation of radiation-induced effects and as an illustration of the integration of electronic and localised state aspects into a common framework. 

The model captures the decreased but sustained activity over platinum promoted sulfated zirconia in the presence of hydrogen by eliminating all contributions to the activity by the S sites and decreasing the activity over the S2 sites by 70%. The deactivation constant for the S2 sites was set to zero to match the experimental data. These results from the model are consistent with a two active site concept where hydrogen inhibits that activity of one type of active sites more than the other type of active site. 

In the following, we present a scenario for CO electrooxidation on Pt nanoparticles to elucidate the role of particle size and surface structure. The model employs the active site concept and highlights the role of finite surface mobility of adsorbed CO. 

Active site concepts for Mn- and Ce-based systems are not yet well developed, which is partly due to problems in the reliable differentiation between oxidation states in case of Mn. 

FIGURE 3.8 Mapping of a 3D nanoparticle model onto a 2D surface model with active and inactive sites. The active site concept is the integral part of the heterogeneous surface model of nanoparticle activity, explored here for CO d electro-oxidation. Immobile adsorbs, preferentially, on specific sites called the active sites. The mobile CO d is found on the remaining inactive catalyst sites. (Reprinted with permission from Andreaus, B.et al. 2006. Kinetic modeling of COad monolayer oxidation on carbon-supported platinum nanoparticles. J. Phys. Chem. B, 110, 21028-21040, Figure 2,7,8,9, American Institute of Physics.)... 

Figure 6.9. The bismetallo active site concept of Buckingham (253). Reprinted with permission. Copyright 1978 by the American Chemical Society.

The main lesson from the analysis given above is that the activation free energy of the reaction is strongly correlated with the stabilization of the ionic resonance structure by the protein-active site. The generality of this concept will be considered in the following chapters. 

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). 

Wei-Ping won the bet. A series of rapid kinetic experiments provided strong support for the concept of two independent active sites. CODH/ACS was reacted with CO and the rate of development of each of the enzyme s characteristic EPR signals was compared with the rates of CO oxidation and acetyl-CoA synthesis. On the basis of these... 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Langmuir s research on how oxygen gas deteriorated the tungsten filaments of light bulbs led to a theory of adsorption that relates the surface concentration of a gas to its pressure above the surface (1915). This, together with Taylor s concept of active sites on the surface of a catalyst, enabled Hinshelwood in around 1927 to formulate the Langmuir-Hinshelwood kinetics that we still use today to describe catalytic reactions. Indeed, research in catalysis was synonymous with kinetic analysis... 

Before deriving the rate equations, we first need to think about the dimensions of the rates. As heterogeneous catalysis involves reactants and products in the three-dimensional space of gases or liquids, but with intermediates on a two-dimensional surface we cannot simply use concentrations as in the case of uncatalyzed reactions. Our choice throughout this book will be to express the macroscopic rate of a catalytic reaction in moles per unit of time. In addition, we will use the microscopic concept of turnover frequency, defined as the number of molecules converted per active site and per unit of time. The macroscopic rate can be seen as a characteristic activity per weight or per volume unit of catalyst in all its complexity with regard to shape, composition, etc., whereas the turnover frequency is a measure of the intrinsic activity of a catalytic site. 

Since early in this century the concept of the active site in catalysis [1] has been a focus of attention in this area of chemistry. This was proposed to be that ensemble of surface atoms/reactants which is responsible for the crucial surface reaction step involved in a catalytic conversion. Since those days much work has been done in the area, which cites the concept of the active site. However, no such ensemble has been positively identified due to the lack of availability of techniques which could image such a structure, which is of atomic dimensions. 

It has been revealed that the formation of protonic acid sites from molecular hydrogen is observable for the catalysts other than Pt/S042--Zr02, and the protonic acid sites thus formed act as catalytically active sites for acid-catalyzed reaction. We propose the concept "molecular hydrogen-originated protonic acid site" as a widely applicable active sites for solid acid catalysts. 

The final method of coupling enzyme reactions to electrochemistry is to immobilize an enzyme directly at the electrode surface. Enzyme electrodes provide the advantages already discussed for immobilization of enzymes. In addition, the transport of enzyme product from the enzyme active site to the electrode surface is greatly enhanced when the enzyme is very near to the electrode. The concept of combining an enzyme reaction with an amperometric probe should offer all of the advantages discussed earlier for ion-selective (potentiometric) electrodes with a much higher sensitivity. In addition, since the response of amperometric electrodes is linear, background can be selected. 

Today, it is accepted that Langley and Ehrlich deserve comparable recognition for the introduction of the receptor concept. In the same years, biochemists studying the relationship between substrate concentration and enzyme velocity had also come to think that enzyme molecules must possess an active site that discriminates among various substrates and inhibitors. As often happens, different strands of evidence had converged to point to a single conclusion. 

The understanding of the catalytic function of enzymes is a prime objective in biomolecular science. In the last decade, significant developments in computational approaches have made quantum chemistry a powerful tool for the study of enzymatic mechanisms. In all applications of quantum chemistry to proteins, a key concept is the active site, i.e. a local region where the chemical reactivity takes place. The concept of the active site makes it possible to scale down large enzymatic systems to models small enough to be handled by accurate quantum chemistry methods. 

In principle, sites a, IT, and c need not be association sites as depicted by Ogston but could be steric sites that form obstructions such that the adsorbed molecule is chirally directed. Only one active site is actually required providing the remaining two sites (protuberances or cavities) are different from each other and from the active site that catalyzes the reaction. They could be identical providing they are not symmetrically oriented with respect to the active site (not an isosceles triangle). These are the basic concepts for a chiral environment on a surface and they lead to the three basic methods for creating chiral surfaces in heterogeneous catalysis. 

NB2001 (23), a prodrug of triclosan, has been developed based on the enzyme-catalyzed therapeutic activation (ECTA) concept. Evidence supporting ring opening of the cephalosporin moiety by penicillin-binding proteins and/ or (3-lactamases to release triclosan at the bacterium site, as well as sub-pg/mL MIC on [3-lactamase-positive strains, demonstrates the potential of this approach [42]. 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

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